A couple of things... First of all, you need at least 3 full tanks before your car is listed in the public database. To get a badge for your signature, go to your vehicle page. Right under the last 10 fuel entries you will see a row of icons. Click on the one that looks like this:
It will take you to a page where you can select a badge for your signature. You don't have to add it each time, just to go your control panel, and add the code from the banner page to your signature.